The present invention relates to the field of thermal transfer recording and, more particularly, to a thermal transfer recording system and a specially configured multi-stripe color transfer ribbon for recording a tonal or grey scale color image on a recording sheet.
The concept of using a thermal transfer ribbon carrying sequential stripes of three or more different color thermally transferable inks thereon for recording a color image on a recording sheet, e.g. plain paper, is well known in the art.
Typically, the ribbon comprises a web having parallel stripes of different color ink on one surface. The stripes, generally extending perpendicular to the web's direction of travel, are disposed in a repeating series or set of four different colors--e.g. yellow, magenta, cyan and black.
The ribbon is adapted to be located between the recording sheet and an oppositely disposed print head of the line printing type comprising a linear array of individually addressable heating elements. With the first ink stripe of a set registered or overlying the print head, selected heating elements are energized causing ink transfer to the recording sheet. The transfered ink forms dots of a first color on the first line of the recording sheet. The size of each dot generally is proportional to the amount of heat applied, i.e., dot size progressively increases with increased amounts of heat applied to form a dot. Next, the ribbon is advanced relative to the print head to register the next color stripe. The recording sheet is held in place so that dots of the second color are recorded over the first set of dots on the same line. The sequence is repeated to overprint the third and fourth ink colors, thus providing a line of pixel defining resultant dots of desired colors in a manner analogous to four-color lithograph printing. Then, the recording sheet is advanced one line position relative to the print head and the four step process of sequential overprinting with each ink color in the set is repeated to record the next line, etc.
For representative examples of thermal color recording systems employing this technique see U.S. Pat. Nos. 4,250,511 and 4,458,253. Also see U.S. Pat. No. 3,726,212 wherein separate different color transfer ribbons are mounted in side by side relation on a linear translatable carriage for sequential presentation at a registered position overlying the print head.
The color image to be recorded is defined by a matrix array of minute pixel areas each of which has a desired color and density or tone generally specified by electronic image signals provided from a computer or video source. The desired color is produced by specifying which of the different ink colors are to be combined by overprinting to produce a resultant dot of a desired color in a given pixel area. The overall tone or density of a given pixel area, on the other hand, is defined by the size of the resultant dot recorded in relation to the overall size of the pixel area. If the resultant dot is printed on white paper and is small compared to the area of the pixel area, the eye will perceive it as being of very light tone. If the resultant dot is larger, e.g., half the size of the pixel area, the eye perceives this to be a pixel area of medium tone or density. To produce the darkest tone or density, the resultant dot is made large enough to substantially fill the pixel area. Thus, perceived tone is adjusted by varying dot size in a manner analogous to half-tone color lithographic printing.
The quality of the recorded image therefore is heavily dependent on how precisely the thermal transfer recording system can control dot size. For color recording, the dot size control problem is especially difficult because to form a resultant dot of a given color it may have to be over printed with up to three different ink colors each of which may have a slightly different size to achieve correct color balance.
The prior art recording systems noted above do not address the issue of varying dot size in a precise manner. As noted earlier, the size of a thermally transfered dot generally will be proportional to the amount of heat applied to an ink stripe to effect ink transfer. If there are variations in the resistivity of the individual print head elements or variations in the heating voltage applied to these elements, the amount of heat generated will vary from element to element and control over dot size is impaired. In addition, there may be variations in the thermal response of different parts of the ribbon which also diminish ability to precisely control dot size.
In my earlier filed, commonly assigned, copending applications U.S. Ser. Nos. (Polaroid cases 7075 and 7076) are disclosed and claimed a closed loop thermal transfer recording system and specially configured thermal transfer ribbon for recording a monochrome tonal or grey scale image on a recording sheet.
The ribbon has a thermally transferable ink layer on the front side which contacts the recording sheet and a thermally sensitive indicator layer on the back side. When heat is applied to the ribbon, an indicator mark is formed on the back side which is proportional to the size of the recorded ink dot. Thus, the density of the ribbon area where the corresponding mark is formed is indicative of the density of the pixel area in which the ink dot is formed. Control over the application of heat is achieved by feedback. The corresponding mark is optically monitored with a photodetector to provide a monitored density signal. This indication of actual density is compared to the electronic image signals specifying desired density. Based on this comparison, the application of heat is regulated to progressively increase dot size until a predetermined comparison value is achieved, whereupon application of heat is terminated.
In the interest of improving the quality of color images produced by thermal transfer recording, there is the need for a thermal transfer recording system and color ribbon which is configured to operate in a closed loop mode for more precisely controlling recorded dot size.
Therefore, it is an object of the present invention to provide a thermal transfer recording system and color thermal transfer ribbon capable of recording high quality color tonal images by providing the ability to more precisely control the size of recorded dots.
It is another object of the invention to provide such a thermal transfer recording system and ribbon for recording a tonal or grey scale color image by printing ink dots of varying color and size on a recording sheet.
Yet another object is to provide such a thermal transfer recording system which operates in closed loop mode and is configured to optically monitor indicator marks on the back side of the ribbon for controlling the size of dots formed from each of a plurality of different color inks disposed in sequential stripes on the front side of the ribbon.
Still another object is to provide such a recording systems including means for efficiently advancing the ribbon with respect to the print head to sequentially present each of the ink stripes in a set at a registered position with respect to the print head.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.